In the production of cast iron, when graphite precipitates in D type, we call it supercooled gray iron. The results show that
Nb also has a great response to supercooled gray iron. _
Pig iron, scrap steel and 75 ferrosilicon were prepared in medium frequency induction furnace, which were divided into 3.0%~3.4% C, 1.2%~1.6% Si, 0.5%~0.9% Mn, P < 0.3%, S < 0.015%, 0.02%~0.09% Nb molten iron, which was poured into wet mould at 1340 ~1360 ~C, and the mechanical properties and metallographic specimens of 10mm were prepared. By metallographic examination, the graphite structure is D-type. It can be seen from metallographic photographs that for the supercooled gray cast iron with D-shape graphite, with the increase of Nb content, the graphite becomes finer. When Nb content reaches 0.05%, the content of Nb is increased and irregular block graphite appears. Niobium has little effect on the structure and morphology of cast iron matrix, and the pearlite grain is slightly refined.
Figs 5 to 7 show the effect of niobium on the mechanical properties of undercooled gray iron. The results show that the tensile strength, hardness and impact toughness increase with the increase of Nb content. When the content of Nb is 0.05%, the mechanical properties reach the maximum value, and continue to increase the content of Nb, the mechanical properties decrease.
